A mechanism is proposed for the formation of guanidinosuccinate, which occurs in the urine from uremic patients. It proposes that aspartate is converted to canaline and then ureidohomoserine. This condenses with a second molecule of aspartate to form canavaninosuccinate. At alkaline pH (8.5) this is reduced by lipoate in the presence of Fe ions and liver to form homoserine and guanidinosuccinate. Alternatively, at acid pH (6.5) canavaninosuccinate decomposes to form canavanine and fumarate. The canavanine then transamidinates to glycine to form guanidinoacetate. This is the normal reaction. In kidney disease, loss of the ability to acidify the urine results in guanidinosuccinate formation. All of the steps listed above have been shown to take place with human liver extract except the formation of canaline from aspartate. It is proposed to explore the mechanism by means of which canaline if formed, probably from aspartate via the semialdehyde or homoserine. For this purpose liver homogenates and extracts will be utilized adding substrate mixtures from which the canaline may be produced. The overall mechanism is also being explored with isolated liver cells. BIBLIOGRAPHIC REFERENCES: Koller, A., Comess, J.D., and Natelson, S., Evidence supporting a proposed mechanism explaining the inverse relationship between guanidinoacetate and guanidinosuccinate in human urine. Clin. Chem. 21, 235 (1975). Sherwin, J.E. and Natelson, S., Serum and erythrocyte argininosuccinate lyase assay by NADH fluorescence generated from formed fumarate. Clin. Chem. 21, 230 (1975).